Method and device for continuously casting ingots, slabs or thin slabs

ABSTRACT

A continuously cast strand is subjected to soft reduction between the guide rollers below the mold and between the soft reduction stretch and the mold the strand is subjected to format or thickness reduction between at least one pair of position controlled guide rollers so that independently of the quality of the steel, the casting speed and the spray cooling, the position of the type of the mold cavity and hence the molten steel therein is maintained constant along the soft reduction stretch.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage of PCT/EP02/04801 filed 2 May 2002and is based upon German national application 101 22 118.5 of 7 May 2001under the International Convention.

FIELD OF THE INVENTION

The invention relates to a method of continuous casting of blooms,ingots, billets, slabs or thin slabs in a continuous casting plant whichhas, beneath a mold, roll pairs which are actuated for example bypath-controlled or position-controlled hydraulic cylinders, in strandguide segments which have their mutual spacing adjustable, whereby thecast strand by a tapered setting of at least one of its strand guidesegments is reduced in thickness over a soft-reducing stretch extendingto a sump or melt-filled cavity tip and in the region of its liquid coreis subjected by at least one roll pair to a size or thickness reductionbetween the mold and the soft-reduction stretch (SR stretch).

BACKGROUND OF THE INVENTION

In continuous casting plants for slabs, to reduce the core porosity andthe core segregation, the soft reduction process (SR process), amongothers, is used.

In bloom, ingot and billet plants, the soft reduction is preferablyeffected in a straight-drive region. The precondition for an improvementof the internal structure quality is that the final solidification ofthe strand be effected in the soft reduction (SR) stretch which is setto be tapered or conical. A final solidification upstream or downstreamof the SR stretch not only does not give rise to any improvement in theinternal quality, but on occasion can result in a detriment to theinternal quality.

The conical or tapered roll setting of the strand guide segments iseffected either as a fixed setting by means of hydraulic cylinders andspacer members or as a flexible setting, for example withposition-controlled hydraulic cylinders.

The so-called operating window of the continuous casting plant, forexample the matching of the casting speed or another casting parameter,the intensity of the spray cooling or the steel quality is given by thenumber of SR roll pairs and the type of adjustment. With an increasingroll count, the operating window can be increased. This increase howeveris not in proportion to the increase in the capital cost. Furthermorethe position of the sump tip or melt-cavity tip can only be influencedto a limited extent by the spray cooling.

In a known process according to the German Patent Document DE 41 38 740A1, the strand can pass through a soft reduction stretch at whose entrythe strand is not yet fully solidified but at whose end the strand issolidified throughout its thickness and for which, among other things,the casting speed is a significant operating parameter. In the region offinal solidification, a thickness reduction is effected, for example forthin slabs, of between 0.5 and 3 mm per meter of the cast length. Forthis purpose in the soft reduction stretch, the roll pairs of individualsegments are brought closer together to follow the shrinkage ratio ofthe strand so as to provide a compaction of the internal or latticestructure in the region of residual solidification and to bring about animprovement in the strand internal quality.

European Patent Document EP 0 834 364 A1 describes a method and a devicefor high speed continuous casting plants with a strand thicknessreduction during the solidification, whereby after the so-called castrolling, the strand cross section is linearly reduced over a minimumlength of the strand travel directly below the mold, this being followedby further strand reduction over the remainder of the strand path, the“soft reduction” until immediately upstream of the final solidificationor the sump or melt-cavity tip. Through these technological features ofthe method, the strand cross section reduction is so carried out that acritical deformation of the strand shell is not exceeded inconsideration of the high casting speed and steel quality.

European Patent Document EP 0 17 77 96 B1 discloses a method ofadvancing and dressing straightening a cast strand in the straighteningand outlet region of an arcuate continuous casting plant, wherebyoppositely disposed rolls are biased by a spring force against theferrostatic pressure of the cast strand and are adjustably spaced fromone another during the casting process. In this manner a cast strandregion of increased solidity is avoided and during the travel of thestrand between opposing rolls the respective spring force can be reducedby a lower counter force.

The slabs or blooms produced in the continuous casting plantscorresponding to the aforementioned documents serve as startingmaterials for rolling milled products for the production of plates orstrip.

OBJECT OF THE INVENTION

The object of the invention is to match or increase the operating windowof a continuous casting plant as to its variable casting parameters andby influencing the sump or melt-cavity tip position to obtain acomparatively uniform and optimal internal or lattice structure in thecast strand.

SUMMARY OF THE INVENTION

To achieve this object the number of roll pairs and their more or lessnarrow adjustment relative to one another in the region of the liquidcore of the cast strand is matched to the casting speed to provide aflexible match of the casting process thereto and thereby a sizethickness reduction of the cast strand with the liquid core so adjustedthat independently of the steel quality, the intensity of the spraycooling and the casting speed, the position of the sump melt-cavity tipin the SR stretch is maintained approximately constant.

As a result, in spite of an increasing casting speed and/or alteredspray cooling, for example, the format thickness is so reduced that theposition of the sump tip melt cavity tip is held practically constant inthe SR stretch. Thus within a large casting speed range with reducedmechanical and technological expenditure, a uniformly good internalquality of the cast strand is ensured.

As a consequence in spite of changing operating parameters during thecasting process, a uniformly optimal lattice structure of the castingstrand is produced and an unnecessarily increased loading of the rollsand their bearings within the strand guide is avoided. The working outof the process provides that the arrangement and/or setting of the rollpairs for the size thickness reduction in the region of the liquid coreof the casting strand is determined by the measurements of the castingcross section or format cross section of the cast strand.

And finally the method according to the invention provides that thethickness reduction is undertaken directly upstream of the SR stretch.

As a result, a matching or enlargement of the operating window of thecontinuous casting plant to various casting parameters is achieved in aneconomical manner and, especially, the influence of the melt-cavity tiplocation is largely rendered independent from the casting parameters inthe sense that the melt-cavity tip always lies in a segment of the SRstretch which contributes to the improvement of the internal quality ofthe cast strand as a consequence, over a large casting speed range auniformly high quality cast strand structure is ensured with a lowexpenditure with respect to equipment and technology.

BRIEF DESCRIPTION OF THE DRAWING

Details, features and advantages of the invention are given in thefollowing explanation of an embodiment schematically illustrated in thedrawing.

The sole FIGURE shows in a purely diagrammatic illustration a strandguide below a mold with a configuration according to the invention aswell as with a soft reduction stretch SR.

The part of the apparatus involving the strand guide according to thedrawing has below the mold a first strand guide segment 2.1 for parallelstrand guidance.

The strand 5, which is only sketched in its illustration below the meltinlet 6 into the mold 1, has an initial crust of the forming strandshell 8.

According to the invention the strand 5 in the region of its liquid core7 between the mold 1 and the soft reduction stretch SR is subjected byat least one roll pair 4, 4′ to a size reduction thickness reduction.The size or format reduction by the roll pair 4, 4′ is followed by astrand guide segment 2.2 for the parallel strand guidance of the caststrand 5. The roll pairs 4, 4′ are each actuated by respectivepath-controlled or position-controlled hydraulic cylinders, not shown ingreater detail so that they can overcome the hydrostatic pressure of themelt and thus impart a local size reduction or thickness reduction instrand 5, e.g. 10 mm/m, in the casting direction upstream of the softreduction stretch SR with a thickness reduction of for example 1 mm/m.In the casting direction below the soft reduction stretch, a furtherstrand guide segment 2.3 for parallel strand guidance is arranged.

The number and setting of the roll pairs 4, 4′ in the region of theliquid core 7 in the cast strand 5 enable a flexible matching of thecasting process to the casting speed or a variation therein.

Because of the thickness reduction of the strand with the liquid core,the size dimensions are continuously so adjusted in dependence upon thecasting parameters like casting speed, steel quality and castingtemperature that the sump tip or melt-cavity tip 9 largely independentlyfrom the aforedescribed casting parameters lies in a segment of the SRstretch which contributes to the improvement in the internal quality ofthe casting strand. With increasing casting speed or reduced spraycooling, for example, the format thickness is so reduced that theposition of the sump tip 9 is held practically constant in the SRstretch. Thus over a large casting speed range with low machine cost andtechnology expense, a uniformly good internal quality of the castproduct is ensured.

1. A method of continuously casting blooms, slabs or thin slabscomprising the steps of: (a) introducing a steel melt into a mold shapedto produce a continuous strand emerging from a bottom of said mold andformed by a progressively solidifying shell of steel surrounding a coreof the molten steel filling a melt cavity in said shell; (b) spraycooling said strand as said strand emerges from said mold in a pluralityof successive strand-guide segments each of which comprises a pair ofmutually spaced sets of guide rollers receiving the strand between them,at least one of said segments spaced below said mold having the sets ofguide rollers thereof tapering toward one another to form asoft-reduction stretch within which solidification of the strand iscompleted over the entire cross section of the strand and in which a tipof the melt cavity is formed; and (c) controlledly reducing a thicknessof said strand between said soft-reduction stretch and said mold betweenrollers of at least one format-reducing roller pair so thatindependently of the quality of the steel, the casting speed and thespray cooling, the position of said tip along the soft-reduction stretchis maintained approximately constant.
 2. The method defined in claim 1wherein the thickness of the strand is controlledly reduced in step (c)between a plurality of said format-reducing roller pairs arranged andset based upon casting cross section of said strand.
 3. The methoddefined in claim 1 wherein the thickness of the strand is controlledlyreduced in step (c) by said format-reducing roller pair directlyupstream of said soft-reduction stretch.
 4. The method defined in claim1 wherein the rollers of said format-reducing roller pair are set withposition control.
 5. The method defined in claim 1 wherein the rollersof said at least one of said segments spaced below said mold having thesets of guide rollers thereof tapering toward one another to form saidsoft-reduction stretch are provided with roller-position control.